I don't think you should put any time constraints on making a skin or wallpaper. If you are doing this for fun,(which I assume you are, as there is no profit to be made for most of us!)just take as long as you need to produce something that YOU would like to use on YOUR desktop. Too many people pump out bland, "template" skins, as I call them, to satisfy their needs as "download junkies", and this ultimately affects overall quality of sites like this.
Being "old school", I am used to quality, artistic skins, and this trend of using the same template with a different gradient across it and calling it a new skin, disappoints me
So, what I am trying to say is, just take as long as you want to make a skin. Essorant, one of the great skinners of the moment recently said to me "It's not a race, is it?", and I agree. I have at least three DesktopX themes that I have been working on for about 3 months now, and I am in no rush to release them, as I am not happy with them yet.